A general ball valve is constituted with a valve body, a ball and a handle. The valve body includes an inflow vent through which a fluid comes in, and an outflow vent through which the fluid passes out. The ball is rotatably disposed in an internal space of the valve body so as to open or close the valve which communicates with the inflow vent and outflow vent. Also, the handle controls the rotation of the ball.
A ring type seat is disposed between the internal space of the valve body and the inflow vent and between the internal space of the valve body and the outflow vent to be tight contact with a corresponding peripheral portion of the ball.
Such a ring type seat is conventionally formed of Teflon resin to obtain gastightness and lubricability.
However, the conventional ball valve seat formed of Teflon resin is soft in characteristic and shows a low antiabrasion. Further, considering a repeated friction in the ball valve between the seat and the ball, the Teflon resin seat tends to be easily worn out and subject to deforming when exposed to a high temperature (more than 150.degree. C.) and a high pressure.